1. Field of the Invention
The present invention relates to a new type of resistance-type oxygen sensor, and relates more specifically to an oxygen sensor for measuring oxygen partial pressure which is used mainly in air-fuel ratio feedback control systems for controlling the air-fuel ratio of automobile exhaust gas in order to improve the purification rate of the exhaust gas as well as fuel consumption for example. In the present invention, new technologies and new products relating to a new type of resistance-type oxygen sensor are provided which is an oxygen sensor having a gas detection part made of an oxide semiconductor, the resistivity of which varies depending on the oxygen partial pressure of the atmospheric gas, and with which a variety of problems with conventional products, such as the high resistivity of oxide semiconductors and high temperature dependence of sensors and the complexity of the circuits, can be fundamentally resolved in the technical field of resistance-type oxygen sensors using oxide semiconductors as oxygen gas detectors which are used as oxygen sensors for automobile exhaust gas.
2. Description of the Related Art
In the past, solid electrolytes have mainly been used as oxygen sensors for automobiles (Japanese Patent Application Laid-open No. S55-137334). This type of sensor measures the difference between the oxygen partial pressures of a reference electrode and a measurement electrode, and always requires a reference electrode. Consequently, the problem with this type of sensor is that the sensor structure is complex, making miniaturization difficult. To resolve this problem, a type of resistance-type oxygen sensor has been developed for example which does not require a reference electrode (Japanese Patent Application Laid-open No. S62-174644).
To give a simple explanation of the measurement principles of this type resistance-type oxygen sensor, first, the oxygen vacancy concentration of the oxide semiconductor varies with the oxygen partial pressure of the atmosphere. In this case, the resistivity or electrical conductivity of the oxide semiconductor is in a 1:1 correlation with the oxygen vacancy concentration, so that the resistivity of the oxide semiconductor changes as the oxygen vacancy concentration changes. Consequently, the oxygen partial pressure of the atmosphere can be known by measuring the resistivity or resistance.
Titanium oxide has conventionally been used as the material for the oxide semiconductor of this resistance-type oxygen sensor, but the problem with this material is that it has poor durability and stability. To fundamentally resolve these problems, the inventors have already researched and developed a resistance-type oxygen sensor using cerium oxide as the oxide semiconductor. This cerium oxide is known to be durable in corrosive gas (see E. B. Varhegyi et al, Sensors and Actuator B, 18-19 (1994) 569). However, a resistance-type oxygen sensor using cerium oxide alone as the oxide semiconductor had the problem that the resistivity thereof was high. If the resistivity is high, the problem that the circuits for measuring the resistance of the oxygen gas detector become more complicated for example is generated. To solve this problem, a resistance-type oxygen sensor was developed using an oxygen gas detection part having a solid solution of cerium oxide added with zirconium oxide as the oxide semiconductor (Japanese Patent Application Laid-open No. 2004-93547).
However, even with this resistance-type oxygen sensor the resistivity of a solid solution of cerium oxide with added zirconium oxide is not low enough, and a new problem is that the resistivity of a solid solution of cerium oxide added with zirconium oxide is actually more temperature dependent than the resistivity of cerium oxide with nothing added. There exists a problem that when temperature dependence is high, output errors increase as the temperature rises.